Merge pull request #151150 from agbrooks/oci-layer-order

dockerTools.buildImage: Fix incorrect layer unpack order before executing runAsRoot script

nixos/tests/docker-tools.nix

@@ -215,6 +215,12 @@ import ./make-test-python.nix ({ pkgs, ... }: {
                 f"docker run --rm  ${examples.layersOrder.imageName} cat /tmp/layer{index}"
             )
 
+    with subtest("Ensure layers unpacked in correct order before runAsRoot runs"):
+        assert "abc" in docker.succeed(
+            "docker load --input='${examples.layersUnpackOrder}'",
+            "docker run --rm ${examples.layersUnpackOrder.imageName} cat /layer-order"
+        )
+
     with subtest("Ensure environment variables are correctly inherited"):
         docker.succeed(
             "docker load --input='${examples.environmentVariables}'"

pkgs/build-support/docker/default.nix

@@ -240,7 +240,7 @@ rec {
           # Unpack all of the parent layers into the image.
           lowerdir=""
           extractionID=0
-          for layerTar in $(tac layer-list); do
+          for layerTar in $(cat layer-list); do
             echo "Unpacking layer $layerTar"
             extractionID=$((extractionID + 1))
 

pkgs/build-support/docker/examples.nix

@@ -405,6 +405,29 @@ rec {
     created = "now";
   };
 
+  # 23. Ensure that layers are unpacked in the correct order before the
+  # runAsRoot script is executed.
+  layersUnpackOrder =
+  let
+    layerOnTopOf = parent: layerName:
+      pkgs.dockerTools.buildImage {
+        name = "layers-unpack-order-${layerName}";
+        tag = "latest";
+        fromImage = parent;
+        contents = [ pkgs.coreutils ];
+        runAsRoot = ''
+          #!${pkgs.runtimeShell}
+          echo -n "${layerName}" >> /layer-order
+        '';
+      };
+    # When executing the runAsRoot script when building layer C, if layer B is
+    # not unpacked on top of layer A, the contents of /layer-order will not be
+    # "ABC".
+    layerA = layerOnTopOf null   "a";
+    layerB = layerOnTopOf layerA "b";
+    layerC = layerOnTopOf layerB "c";
+  in layerC;
+
   # buildImage without explicit tag
   bashNoTag = pkgs.dockerTools.buildImage {
     name = "bash-no-tag";